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corundum (n.)

"very hard mineral" (crystalline aluminum oxide) used for grinding and polishing other gems, steel, etc., 1728, from Anglo-Indian, from Tamil (Dravidian) kurundam "ruby sapphire" (Sanskrit kuruvinda), which is of unknown origin. It is a dull or opaque variety of sapphire, amethyst, ruby, and topaz; in hardness it is next to diamond.